This paper written by Adiel Schremer seems to be a solid treatment of this topic. He recognises that it is impossible to determine the number of polygamous marriages in ancient times. However this is not a necessary precondition to answering the question at hand. Rather,
A polygynous society is best defined as not monogamous, and a monogamous society is a society in which an enforced norm of monogamy exists.’ According to such a definition, a society which allows its male members to marry more than one woman and in which cases of polygyny are actually found – even if only limited cases – should be regarded polygynous. This classification does not reflect any statistical claim, nor does it contain any statement regarding the family life of most of that society’s members. Rather, it intends primarily to express something about the society’s attitude towards marriage and family life.
His conclusions are:
On the basis of the available sources no precise answer can be given to this question. To determine the exact percentage of men married to more than one woman at once is simply not possible, and so perhaps the answer to this question will never be known. However, it seems that the inability to provide an exact number is not the real issue. What is important, and particularly in light of the definition suggested at the beginning of this paper, is that Jewish society of Eretz Israel of the Second Temple, the Mishnaic, and Talmudic periods may be designated as a polygynous society. This conclusion is based, on the one hand, on the fact the rabbis did not prohibit polygyny, and, on the other, on the evidence indicating the actual practice of polygyny during these periods.
There are alongside these sources a few Midrashic passages which appear to express a negative view of polygyny. Scholars based their claim regarding monogamous trends in Jewish society of the period under discussion on these sources. However, a close reading of the latter shows that the rabbis did not oppose polygyny in itself. As long as polygyny did not threaten daily family life and was motivated by the wish to bear children (or any other positive motive), the rabbis did not regard it as a negative act. It appears that rabbinic views of the institution of marriage were not guided by an ethical or theological sense of that institution, but by a practical approach. As Herr put it: “Marriage, for the masters of Halakhah, was not considered as a ‘holy’ goal, but only as an optimal socio-economic means.”